NullPointerException与ImageButton允许dropDownMenu可显示

Question

我的应用程序将正常加载，并且能够登录。单击登录后，应用程序将冻结并崩溃，并且此错误将出现在我的IDE中。我似乎在Dashboard.java的第51行上找不到nullPointerException的来源：

"btnAccount.setOnClickListener(new View.OnClickListener()"

在构建此应用程序时，如果有帮助，请按照page中的说明进行操作。您也可以忽略与TextView Logout关联的重复代码，这是出于测试目的，我想将其保留在代码中，因为我想为所有人提供完整的文件。

这是我的Dashboard.java文件：

public class Dashboard extends AppCompatActivity {

TextView Logout;
ImageButton btnSubmitIssue;
ImageButton btnCommunity;
ImageButton btnShop;
ImageButton btnHeatMap;
ImageButton btnEncyclopedia;
@Override
protected void onCreate(Bundle savedInstanceState)
{
    ImageButton btnAccount = findViewById(R.id.btnAccount);
    final PopupMenu dropDownAccount = new PopupMenu(getApplicationContext(), btnAccount);
    final Menu menu = dropDownAccount.getMenu();
    menu.add(0,0,0,"Account Details");
    menu.add(0,1,0,"Change Password");
    menu.add(0,2,0,"Logout");
    dropDownAccount.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {
        @Override
        public boolean onMenuItemClick(MenuItem item) {
            switch(item.getItemId())
            {
                case 0:
                    startActivity(new Intent(Dashboard.this , placeholder.class));
                    return true;
                case 1:
                    startActivity(new Intent(Dashboard.this, placeholder.class));
                    return true;
                case 2:
                    Intent intent2 = new Intent(Dashboard.this, Login.class);
                    startActivity(intent2);
                    Toast.makeText(Dashboard.this," Logged out Successfully !", Toast.LENGTH_SHORT).show();
            }
            return false;
        }
    });
    btnAccount.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dropDownAccount.show();
        }
    });
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dashboard);
    btnCommunity = findViewById(R.id.btnCommunity);
    btnSubmitIssue = findViewById(R.id.btnSubmitIssue);
    btnShop = findViewById(R.id.btnShop);
    btnHeatMap = findViewById(R.id.btnHeatMap);
    btnEncyclopedia = findViewById(R.id.btnEncyclopedia);
    // Will setup button Listeners once the pages for these have been created.
    btnEncyclopedia.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnHeatMap.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnSubmitIssue.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnShop.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnCommunity.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    Logout = (TextView)findViewById(R.id.tvLogout);
    Logout.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(Dashboard.this, Login.class);
            startActivity(intent);
            Toast.makeText(Dashboard.this," Logged out Successfully !", Toast.LENGTH_SHORT).show();
        }
    });

}
}

这是我的错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.vanda.agrivolution, PID: 10650
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.vanda.agrivolution/com.example.vanda.agrivolution.Dashboard}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.vanda.agrivolution.Dashboard.onCreate(Dashboard.java:51)
    at android.app.Activity.performCreate(Activity.java:7136)
    at android.app.Activity.performCreate(Activity.java:7127)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:6669) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

Answer 1

我自己发现了该错误...有趣的是，一旦您发布问题并再看一遍，您便能够自己解决该错误。

我的错误是某些代码本应位于onCreate方法的顶部。以下行：

super.onCreate(savedInstanceState);
setContentView(R.layout.activity_dashboard);

低于大部分代码。一旦将其移回到onCreate方法的顶部，应用程序就会按预期工作。

这是Dashboard.java文件中我的新onCreate方法（没有其他更改）：

protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dashboard);
    ImageButton btnAccount = findViewById(R.id.btnAccount);
    final PopupMenu dropDownAccount = new PopupMenu(getApplicationContext(), btnAccount);
    final Menu menu = dropDownAccount.getMenu();
    menu.add(0,0,0,"Account Details");
    menu.add(0,1,1,"Change Password");
    menu.add(0,2,2,"Logout");
    dropDownAccount.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {
        @Override
        public boolean onMenuItemClick(MenuItem item) {
            switch(item.getItemId())
            {
                case 0:
                    startActivity(new Intent(Dashboard.this , placeholder.class));
                    return true;
                case 1:
                    startActivity(new Intent(Dashboard.this, placeholder.class));
                    return true;
                case 2:
                    Intent intent2 = new Intent(Dashboard.this, Login.class);
                    startActivity(intent2);
                    Toast.makeText(Dashboard.this," Logged out Successfully !", Toast.LENGTH_SHORT).show();
            }
            return false;
        }
    });
    btnAccount.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            dropDownAccount.show();
        }
    });
    btnCommunity = findViewById(R.id.btnCommunity);
    btnSubmitIssue = findViewById(R.id.btnSubmitIssue);
    btnShop = findViewById(R.id.btnShop);
    btnHeatMap = findViewById(R.id.btnHeatMap);
    btnEncyclopedia = findViewById(R.id.btnEncyclopedia);
    // Will setup button Listeners once the pages for these have been created.
    btnEncyclopedia.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnHeatMap.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnSubmitIssue.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnShop.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    btnCommunity.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(Dashboard.this, placeholder.class));
        }
    });
    Logout = (TextView)findViewById(R.id.tvLogout);
    Logout.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(Dashboard.this, Login.class);
            startActivity(intent);
            Toast.makeText(Dashboard.this," Logged out Successfully !", Toast.LENGTH_SHORT).show();
        }
    });

}